This invention relates in general to xerographic duplicating systems and more specifically to the development of a latent electrostatic image formed on a coated photoconductive paper by applying a polarized magnetically attractable, one component toner.
Many techniques are known for developing a latent electrostatic image with a powdered developer or toner material that adheres to the image pattern and thereby renders it visible. Some of the more common techniques include cascade development, magnetic brush development, development utilizing Van der Waal's forces as described in U.S. Pat. No. 3,166,432 to Gundlach, and liquid toner systems. Each technique has advantages and disadvantages, as discussed in U.S. Pat. No. 3,909,258 to Kotz, with the particular technique selected depending on factors such as the characteristics of the recording medium and the toner and the desired qualities of the developed image.
The cascade technique typically utilizes a two part toner of finely divided pigmented insulating particles carried on relatively large granular materials such as glass beads. The carrier material develops a triboelectric charge on the pigmented material (thereby polarizing the material) that is opposite in polarity to that of the image pattern on the recording medium. When this two part toner material is cascaded over the recording medium under the influence of gravity, the pigmented particles are drawn from the carrier beads to the latent image by an electrostatic attraction. Liquid toner techniques utilize a highly electrically insulating liquid as a carrier material for the pigmented particles. Both of these techniques suffer from the problems inherent in a two component toner where one part is continuously depleted. Other problems involve handling liquids and charged powders cascading under the influence of gravity.
U.S. Pat. No. 3,166,432 to Gundlach describes a development technique where a one part toner is held on a potential biased support member by Van der Waal's forces. Although this technique has the advantage of using a one component toner and produces copies with well filled solid areas, the Van der Waal's forces are weak and tend to vary between batches of the toner. Also, the deposit rate is sensitive to small irregularities in the support surface and changes in environmental factors such as the temperature and humidity.
Another technique applies the toner with a brush, or simulates a brush with a magnet that draws the developer particles across the recording surface. U.S. Pat. No. 2,846,333 to Wilson describes an early magnetic brush applicator in which a series of magnetic discs are mounted on a common rotating shaft in a skewed alignment with respect to the axis of rotation of the shaft. As the shaft is rotated, the shifting magnetic field generated by the discs brushes a magnetic developer material across the image bearing surface of a recording medium. U.S. Pat. No. 3,015,305 to Hall et al. describes a variation of the Wilson magnetic brush where a nonmagnetic insulating sleeve surrounds the rotating discs. This sleeve avoids the deposit of toner due to a current flow through the mass of developer material. As a result, the image has a sharp contrast and the development process is less time sensitive.
U.S. Pat. No. 3,455,276 to Anderson discloses another magnetic brush applicator that utilizes a sleeve that surrounds the magnet. In Anderson, although the sleeve can be nonconductive, it is described principally as a conductive element that can be maintained at a bias potential with respect to the recording surface. Also, Anderson uses a plurality of magnets aligned about a central rotating shaft of a magnetically permeable material. This segmented magnet structure achieves a generally uniform magnetic field along lines parallel to the axis of rotation of the magnet, with lines of equal force being angularly spaced in correspondence with the planes of abutment between adjacent segments of the magnet. As in Wilson and Hall, the toner is brushed into direct contact with the recording surface (there is no air gap between the toner and the recording surface during the deposit). In the Anderson developing system, the deposit occurs in a direction transverse to the direction of the force of gravity.
In contrast to these magnetic brush techniques that draw the developer material over the recording surface, U.S. Pat. No. 3,909,258 to Kotz discloses an applicator that utilizes a rotating segmented magnet housed within a sleeve, but the development process depends on the conduction of an electrical current along chains of a conductive developer material that bridge an air gap between the recording surface and the sleeve. The Kotz system therefore requires that the sleeve be conductive. Also, the deposit of developer material is a function of the electrical potential of the sleeve. This potential is either an applied bias potential, an induced potential due to the current flow, or a combination of applied and induced voltages. Deposit of the developer material, as well as the rate and uniformity of deposit, depend on a careful balance of the forces exerted on the developer material by the magnet, the latent image pattern, and the sleeve. Further, this development technique is relatively sensitive to time since the period of time in which the developer material remains in the nip region between the sleeve and the recording material is very important.
It is therefore a principal object of the invention to provide a system for developing a latent electrostatic image pattern carried in a photoconductive layer coated on paper with a dry, one component toner material carried on a support member in a uniform layer, and spaced uniformly over the latent image pattern, without an electrical flow between the paper and the support member.
Another object of the invention is to provide a development system that reduces the variables effecting the rate and uniformity of deposit, has a good degree of exposure latitude, and affords good control over the toner material.
A further object is to provide a system for high quality development of latent electrostatic images on coated photoconductive paper with a dry, one component toner.